Most LED chip manufacturers do not offer chips larger than 1×1 mm. The light flux that can be produced by each such chip is ever increasing, but still for many applications a single LED chip does not produce enough light, and therefore LED sources with a multitude of chips are produced. In other cases chips of different colors (including different phosphor-converted chips to add shades of white, yellow or green) are combined to allow the production of color-switchable LED light sources or high color rendering white LED light sources.
These LED light sources consist of multiple chips arranged on a planar substrate with or without significant spacing between them. Typical layouts are 2×2 or 3×3 chips arranged in a close-packed square or rectangle, or any higher number of chips, e.g., in the range of 25. Even much higher numbers of chips, often smaller than 1×1 mm, are used for certain LED products.
In all cases the full array of chips is covered with a dielectric material for protection and to enhance light extraction. In some cases phosphors for white light creation are added. Bulk diffusion is sometimes employed too, aiding color mixing. These LEDs can have a total flux of several thousand Lumens.
Two problems arise with these light sources. The first problem is that, when looking back into the source, the user perceives the individual chips as separated bright spots. This may be acceptable if all chips have the same color, but in applications where chips of different colors are mixed (red, green and blue, for example) this is often unacceptable. The second problem arises when secondary optics are added to these multichip LED sources: many optics, especially those that create high collimation, reproduce the structure of the light source in the far field intensity pattern or in the illuminance pattern on a target surface, depending on what they are designed for. That leads to visible brightness variations, or even worse, to color shifts in the pattern that, in the worst case of an RGB LED, can reproduce clearly the different color chips employed.
Diffusers or tessellations on refractive or reflective surfaces are often employed to mitigate these problems although at the expense of some efficiency loss. Furthermore, the diffusion angle that needs to be added to the optical system to erase color or brightness artifacts typically has to be several times larger than the angular range of the collimation that is produced by the secondary optics so that the collimating effect of the secondary optics is partially eroded.
One possible solution to this problem is to employ Köhler integrating optics, and more precisely so-called “fly's eye array” arrays of pairs of microlenses, to erase color and brightness artifacts from a light pattern. Some disadvantages of the “fly's eye array” are that: the microlens arrays must typically be very large to cover the entire exit aperture of the collimation optics; they only work for highly collimated light; they are costly to manufacture; and the light pattern produced is usually a flat-top radiation pattern that is very different from typical smooth radiation patterns wanted in general illumination. Additionally, fly's eye arrays can themselves introduce square or rectangular artifacts, depending on the outline of the individual microlenses they are made of.